There are a large number of applications, for example, in the chemistry, biology, biotechnology, pharmacy and foods technology fields, in which liquids are used as solvents, as starting materials for a process or as reaction products. In many of these applications, the presence of dissolved gases and the therewith connected, possible forming of gas bubbles are undesirable. The forming of gas bubbles can, for example, interfere in the case of the chromatographic isolation of reaction products. Also, measurements of physical or chemical, measured variables in the liquids can be corrupted by the presence of gases or the forming of gas bubbles. Not only in laboratory methods but also in industrial processes, a precise and reliable measuring of certain measured variables is of great importance for both open and closed loop control of the processes and for quality assurance of the products.
In process measurements technology and in environmental technology, analytical devices are frequently applied for determining a measured variable of a liquid. For example, analytical devices can be applied for monitoring and optimizing the cleaning effectiveness of a clarification plant, for monitoring drinking water or for quality monitoring of food. Measured and monitored is, for example, the content of a certain substance (also referred to as the analyte) in the liquid sample. Analytes can include, for example, ions, such as ammonium, phosphate, silicate or nitrate, biological or biochemical compounds, e.g. hormones, or even microorganisms. Other measured variables, which are determined by analytical devices in process measurements technology, especially in the field of monitoring water, include the organic carbon content (TOC=total organic carbon) and the chemical oxygen demand (COD). Analytical devices can be embodied, for example, as cabinet devices or as buoys.
Frequently in analytical devices, the sample to be analyzed is treated by mixing with one or more reagents, so that a chemical reaction can occur in the liquid sample. In a number of analytical methods, the reagents are so selected that the chemical reaction is detectable by means of physical methods, for example, by optical measurements, or by means of potentiometric or amperometric sensors or by measuring conductivity. For example, the chemical reaction can bring about a coloring of the liquid sample, or a color change, which is detectable with optical means. The color intensity is, here, a measure for the measured variable to be determined. The measured variable can, in this case, be ascertained, for example, photometrically. Thus, electromagnetic radiation, for example, visible light, is radiated from a radiation source into the liquid sample and, after transmission through the liquid sample, is received by a suitable receiver. The receiver produces a measurement signal dependent on the intensity of the received radiation, from which the measured variable can be derived.
In order to use such analytical methods in an automated fashion, for example, for industrial applications or for monitoring a clarification plant or a body of water in the outdoors, it is desirable to provide an analytical device, which performs the required analytical method in an automated fashion. The most important requirements for such an analytical device are, besides a sufficient accuracy of measurement, robustness, simple serviceability and the assurance of a sufficient working, and environmental safety.
Semiautomatic and automatic analytical devices are known from the state of the art. Thus, for example, DE 102 22 822 A1, DE 102 20 829 A1 and DE 10 2009 029305 A1 disclose online-analyzers for analyzing samples. These online-analyzers are embodied, in each case, as cabinet devices, which include a control unit, supply containers for reagents, standards and cleaning liquids, pumps for transporting and dosing liquid samples, and the one or more reagents, into a measuring cell, and measuring transducers for optical measurements on the liquid sample treated with the one or more reagents in the measuring cell. The reagents, standards or cleaning liquids are transported from the supply containers and into the measuring cell. Correspondingly, used liquid is transferred from the measuring cell into a waste container.
The liquid sample to be analyzed contains, as a rule, dissolved gases, for example, air, or air components, such as oxygen, carbon dioxide and/or nitrogen. As a result of temperature- or pH changes of the liquid sample during the analytical method or due to chemical reactions when treating the liquid sample with reagents, such dissolved gases can form disturbing gas bubbles. Also, the reagents added to the liquid sample in the analytical device can contain dissolved gases and contribute in equal manner to gas bubble formation.
The presence of dissolved gases, or gas bubble formation, in the liquid sample can corrupt the analytical result ascertained by an analytical device. This is especially true in the case of the described photometric analytical method, in the case of which the liquid sample, pretreated by addition of reagents, and, in given cases, colored, absorbs light. The corruption results from gas bubbles present in the beam path of the measuring radiation radiated through the liquid sample.
In some analytical methods, gas is formed in a liquid sample treatment reaction between a reagent and the analyte or another chemical component of the liquid sample. This can serve for removing disturbing substances from the liquid sample. An example of this is the driving out of disturbing chloride ions from an aqueous liquid sample by the addition of concentrated sulfuric acid before determining the chemical oxygen demand of the sample by means of oxidation by potassium dichromate. This method is described, for example, in DE 10 2009 028165 A1. In the case of these methods, it is likewise of great importance for assuring a sufficient accuracy of measurement that the formed gases are removed from the liquid sample as completely as possible.
A gaseous component driven from the liquid sample can be, in other analytical methods, also a reaction product of the analyte. For example, a liquid sample can be mixed with lye for ascertaining the ammonium content. This converts ammonium to gaseous ammonia and, based on the arising amount of gas, the ammonium concentration of the liquid sample is deduced.
In the case of determining the organic carbon content of a liquid sample, frequently, the inorganic carbon fraction is driven out as carbon dioxide (CO2), by acidification of the liquid sample, before the organic carbon fraction in the remaining liquid sample is oxidized to CO2. A carrier gas stream is then fed through and the TOC content determined from the CO2 concentration in the carrier gas stream. Analytical devices for determining the total carbon content, the TOC content and/or the TIC content are known, for example, from DE 10 2008 025 877 A1, DE 10 2006 058 051 A1 or U.S. Pat. No. 5,340,542. Many TOC analytical devices also determine the inorganic carbon fraction (so-called TIC=total inorganic carbon) based on the amount of CO2 arising from the inorganic carbon compounds of the liquid sample. Also in the case of these methods, it is important, for reaching a sufficient accuracy of measurement, that the gaseous component is separated as quantitatively as possible from the liquid sample, since, otherwise, an analyte concentration is ascertained, which is too low.
For removing gases from liquids used in laboratory applications, in process- and/or analytical technologies or from liquids obtained by chemical methods, often degassing apparatuses are applied. These are sometimes referred to as “degassers” or “debubblers”.
Such an apparatus is described, for example, in U.S. Pat. No. 7,144,443 B2. The apparatus is integrated into a liquid carrying line and includes a tubular, gas- and liquid tight, outer jacket and a tubular, gas transmissive, inner line. The outer jacket has in its wall a connection to a vacuum source, which enables reduction of the pressure reigning in the annular space formed between the inner line and the outer jacket relative to the pressure reigning in the inner line. For degassing a liquid, such is transported through the inner line, while the vacuum source is applied to the space formed between the inner line and the outer jacket. Based on Henry's law, in this way, gas present in solution migrates through the wall of the inner line into the gas phase in the jacket and the liquid is, thus, degassed.
Disadvantageous in this procedure is, however, that the degassing apparatus requires, supplementally to a pump required for transporting the liquid through the inner line, a vacuum source, as well as a special separating diaphragm for the removal of the gas from the liquid. In many applications, such a method is not practical, not least of all in the above described analytical devices, which, depending on area of application, for example, in a clarification plant or at an environmental, analytical measuring point, should be able to function, as much as possible, without other peripheral devices, such as e.g., vacuum pumps.